Method for cleaning tar-bearing waste water and apparatus for performing said method

ABSTRACT

In a method and an apparatus for cleaning tar-bearing waste water ( 17 ), a mixture of water and hydrocarbons, e.g. comprising polyaromatic hydrocarbons and phenols, the mixture is separated into a low-boiling-point part and a high-boiling-point part, bringing the low-boiling-point part on vapour form in a boiler ( 1 ), and the low-boiling-point part is cracked in vapour form at a high temperature in a reactor ( 2 ), providing light combustible gases, which can be utilised in e.g. gas engines, gas turbines or the like. Furthermore, the high-boiling-point part may be used for energy supply to the process or other processes or as an alternative be cracked for providing light combustible gases.

TECHNICAL FIELD

The present invention relates to a method for cleaning tar-bearing wastewater and an apparatus for performing the method.

BACKGROUND ART

In gas production, based on gasification of biomass, coal, etc., it iswell-known that the produced gas contains tar, which in connection withthe use of the gas in e.g. internal combustion engines or gas turbines,has to be removed from the produced gas. The traditional way of removingthe tar is by cooling the gas, whereby the tar and possible water iscondensed, leaving a relatively clean gas and a separated mixture ofwater and tar.

The mixture of water and tar cannot be disposed of to the environmentdue to the content of tar, including polyaromatic hydrocarbons andphenols, which are considered environmentally unsafe, possiblycarcinogenic, poisonous, etc. Furthermore, the acidity of the mixturemay also constitute an environmental problem.

It has been suggested to use ultraviolet light-induced wet oxidation oradsorption on various coke sorbents to clean the waste water. However,electric energy consumption is relatively high for the ultravioletlight-induced wet oxidation and the adsorption has certain limitationsdue to the presence of non-adsorbable compounds.

It has been suggested (Swedish application 402214) that the waste wateris evaporated and separated into a combustible tar fraction and a (moreor less) clean steam fraction. The combustible fraction is subsequentlyburned (oxidised) to provide heat for the evaporation process—either bydirect contact or through a heat exchanger. However, in this way thecalorific value of the tar is converted directly to heat, which severelylimits the utilisation in high efficiency power producingmachinery—specifically gas engines and gas turbines. Therefore, theapplication of the technology suggested will be limited to steamturbines and other equipment based on external firing.

DISCLOSURE OF THE INVENTION

It is the object of the present invention to provide a method forcleaning tar-bearing waste water and an apparatus for performing saidmethod of the kind referred to above, with which it is possible toprovide a high efficiency of the process. By high efficiency is meanthigh cleaning capability and also efficient recovery of the energeticcontent in the tar contaminants for direct use in a gas-engine or gasturbine. This also implies, that in contrast with Swedish application402214—where the contaminants are oxidised—the present inventiondescribes a cracking process conducted under reducing conditions, wherethe high molecular tars and acids are transformed into light combustiblegases which can be utilised in e.g. gas engines, gas turbines or thelike. This object is achieved with a method for cleaning tar-bearingwaste water and an apparatus for performing said method of said kind,which according to the present invention also comprises the features setforth hereinafter. With this arrangement, a relatively clean part of thewaste water on vapour form is cracked at a high temperature providingenergy containing light combustible gases for use in e.g. gas engines,gas turbines or the like, and at the same time a high-boiling-point partof the waste water is provided, which is a combustible concentrate ofthe waste water, which may be used for heating purposes. Preferredembodiments of the method and apparatus are revealed hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed part of the present description, the inventionwill be explained in more detail with reference to the exemplaryembodiments of an apparatus for performing the method in accordance withthe present invention shown in the drawings, in which

FIG. 1 schematically shows an apparatus in accordance with the presentinvention, and

FIG. 2 schematically shows a flow scheme of an alternative apparatus inaccordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The apparatus shown in FIG. 1 is suited for cleaning tar-bearing wastewater, e.g. coming from a biomass gasification unit, i.a. as describedin EP-A-953,627. The apparatus comprises an evaporator 1 receiving thetar-containing water 17 at the bottom for completely evaporation in theevaporator 1. Before leaving the evaporator 1, a high-boiling-point partis separated by means of a droplet separator 7, said high-boiling-pointpart being pumped away by means of a concentrate pump 8. Thelow-boiling-point part is moved onwards by a high-pressure fan 6 feedinga heat exchanger 3, in which the low-boiling-point part is heated up toa relatively high temperature before entering the reactor 2, in whichthe low-boiling-point part is cracked in order to reduce its contents ofhydrocarbons to an environmentally acceptable level, said crackingreactor 2 comprising a burner 5 supplied with concentrate from the pump8 and combustion air 16, thus heating up the reactor 2 by burning off atleast part of the concentrate separated out by means of the dropletseparator 7. The cracked product leaving the reactor 2 is fed downwardsin the heat exchanger 3 whereby it is cooled down by the heat exchangewith the low-boiling-point part in counterflow. The cooled down crackedproduct 4 leaving the heat exchanger 3 is split up into two portions,one fed to the evaporator 1 providing heat for evaporation of thetar-containing water, whereby this part of the cracked product 4 iscondensed and leaves the evaporator 1 in the form of relatively cleanwater 18, sufficiently clean to be led to the sewer system. Another partof the cracked product 4 leaving the heat exchanger 3 can be used asgasification medium in the associated gasifier as indicated at 20.

An alternative apparatus in accordance with the invention is shown inFIG. 2 in the form of a flow scheme, in which parts corresponding to theparts shown in FIG. 1 are supplied with corresponding numbers. Theapparatus shown in FIG. 1 again comprises an evaporator 1, which issupplied with tar-containing water 17 and which evaporates thistar-containing water using a closed loop of pressurised hot water forthe heating, said hot water being heated by means of exhaust 10 from oneor more engines, e.g. driven by gas from a biomass gasification plant,said heat being extracted from the exhausted gas in an exhaust boiler 9,said heating being supplemented by heat extracted from the crackedvapour 4 leaving the heat exchanger 3, extracting said heat in aseparate heat exchanger 12. The evaporator 1 again ends up in aseparator 7 separating out a concentrate which is delivered to aconcentrate buffer tank 14 from which it can be pumped up to a burner 5for this concentrate by means of a concentrate pump 8. Thelow-boiling-point part leaving the evaporator 1 after the separator 7 issubjected to heat exchange with cracked vapour from the reactor 2 in aheat exchanger 3, said heat exchanger 3 further comprising separatechannels for pre-heating air 15 for use in the reactor 2. The reactor 2is heated by means of the burner 5 supplied with air 16 for burning theconcentrate from the buffer tank 14 and further heat is supplied byparts of the low-boiling-point part burning when coming into contactwith the preheated air 15 in a first part of the reactor 2. The hightemperature in the reactor 2 provides a cracking of the mixture suppliedthereto and the high temperature cracked vapour leaving the reactor 2supplies heat to the low-boiling-point part from the evaporator 1 andthe air 15 for the reactor in the heat exchanger 3. The cracked vapour 4leaving the heat exchanger 3 is supplied to the heat exchanger 12 inorder to deliver energy to the hot water pressurised closed loop forheating the evaporator 1. After leaving the heat exchanger 12 the vapourcan furthermore deliver energy to a district heating loop 19 using aheat exchanger 13 in which parts of the cracked vapour may condensateand other parts, e.g. non-condensable, light combustible parts of thecracked product from the reactor 2, are fed to a gas engine or gasturbine 22.

In a preferred process in the system shown in FIG. 2, the hot waterpressurised loop extracts heat from the engine exhaust 10 in the exhaustboiler 9, whereby the water is heated up to a temperature of about 120°C. and this heat is transferred to the incoming tar-containing water 17in the evaporator 1 resulting in vapour at the top of the evaporator ata temperature of about 107° C. In the separator 7, a liquid concentrateis separated and collected in a concentrate buffer tank 14 leaving arelatively clean steam, which is heated in the heat exchanger 3 to atemperature of about 450° C. in counterflow with cracked product andcombustion gases leaving the reactor 2. In the heat exchanger 3, air oran inert gas 15, which may be preheated by passing through themantle-cooling conduct for the heat exchanger 3, is heated to about thesame temperature as the steam, when entering a first part of the reactor2. The inert gas could be provided in the form of cooled exhaust gas 11.The possible hot air spontaneously ignites some of the organic compoundsin this first part of the reactor 2 and further heat is added in orderto raise the temperature in the reactor to about 800° C. The energy forthis is provided by burning concentrate from the concentrate buffer tank14 in the burner 5.

EXAMPLE

In connection with the cleaning of tar-containing water in connectionwith a gasifier, the following cleaning capabilities apply:

Typical Inlet Conditions

-   Acetic acid 14,200 mg/litre-   formic acid 1900 mg/litre-   causing an acidity of pH=2.03-   phenol 730 mg/litre-   guaiacol 1030 mg/litre-   dehydroxy-benzen 1 1400 mg/litre-   other phenols 2840 mg/litre-   and further the PAH's:-   naphthalene 0.45 mg/litre-   antracene/pheantrene <0.005 mg/litre-   the total organic carbon content (TOC) is 45,900 mg/litre

Typical Exit Conditions

The original contaminated water is separated into two streams:

A heavily contaminated fraction (about 10%) with a TOC of about 300,000mg/litre and gross calorific value of about 13 MJ/kg (65-75% of thiswill be reused internally in the process—the rest may be burned inauxiliary boilers in the plant during district heating peak loads).

A clean condensate with a TOC below 15 mg/litre, a total phenol contentbelow 0.15 mg/litre and an acidity of pH=6.90 to 7.10 (therebyeliminating the need of neutralisation).

EXAMPLE 2

In a plant corresponding to FIG. 1, 1266 kg/hour of waste water isboiled in the evaporator 1 heated on the outside using clean steam 4 ata temperature of about 550° C. and a pressure of 102 kPa leaving theheat exchanger 3. The waste water leaves the evaporator 1 as:

steam 1152 kg/hour at about 97° C. and 100 kPa, which after the dropletseparator 7 is compressed in a high-pressure fan 6 to about 105° C. and105 kPa, a combustible concentrate 114 kg/h of which the major fractionis used in the process at the burner 5 as described above. The steampart is heated in the heat exchanger 3 in counterflow with steam leavingthe reactor 2 to about 380° C. and 104 kPa. After the heat exchanger,the temperature is raised to 800° C. using about 81 kg/h of thecombustible concentrate burned off in the burner 5. Based on 0.305kg/MJ, about 320 kg/h air 16 is used at this point. In the FIG. 1apparatus, the burner 5 burns directly Inside the reactor 2 therebypromoting turbulence and elimination of remaining tar traces in thesteam. The steam leaving the reactor 2 will have a flue gas content ofabout 22%, which will reduce the performance of the steam-heatedevaporator due to the presence of inert gases. These inert gases willhave to be withdrawn from the top shell part of the evaporator 1 inorder to improve condensation heat transfer to the evaporator. Thepurified water 18 leaving the evaporator 1 amounts to about 90% of thetar-containing water input 17.

The purified water has a TOC of about 14 mg/litre and contains about 0.4mg/litre phenol, where regulations require below 15 mg/litre.

Although the invention above has been described in connection withpreferred embodiments thereof, it will be evident for a man skilled inthe art that several modifications are possible within the scope of theinvention.

EXAMPLE 3

By operating the evaporator in a way to admit a larger fraction oforganic contaminants in the steam from the evaporator or alternatively(or additionally) inject part of or all of the separatedtar-contaminants from the separator directly into the reaction chamber,which is being held at low stoechiometric conditions, the tar componentswill be cracked into lighter combustible gases. When the tar-watercleaning system described is used in connection with a gasifier thesegases might subsequently be added to the gas cleaning system of theassociated gasifier and increase overall power efficiency.

1. Apparatus for cleaning tar-bearing waste water, e.g. a waste mixtureof water and hydrocarbons, e.g. polyaromatic hydrocarbons and phenols,comprising: a boiler or evaporator for evaporation of the waste mixture,a separator which receives the vaporized waste mixture from said boileror evaporator and which separates the vaporized waste mixture into ahigh-boiling-point part and a remaining evaporated part, a reactor whichreceives the remaining evaporated part and which cracks the remainingevaporated part into a cracked mixture, a heat exchanger located betweenthe boiler or evaporator and the reactor for performing a heat exchangebetween the remaining evaporated part flowing towards the reactor andthe cracked mixture leaving the reactor, and a connection feeding thecracked mixture leaving the reactor and the heat exchanger to the boileror evaporator, thereby extracting heat energy from the cracked mixturefor the evaporation of the waste mixture.
 2. Apparatus in accordancewith claim 1, further comprising a means for a controlled adding of apart of the separated high-boiling-point part to the reactor. 3.Apparatus in accordance with claim 1, further comprising a separatereactor for cracking the high-boiling-point part of the mixture. 4.Apparatus in accordance with claim 1, wherein the reactor comprises aburner in which the separated high-boiling-point part is burned and fromwhich heat is supplied to the reactor.
 5. Apparatus in accordance withclaim 1, further comprising a high-pressure fan for increasing thepressure in the reactor and/or for decreasing the pressure in the boileror evaporator.
 6. Apparatus in accordance with claim 1, wherein theseparator is a droplet separator located at the outlet from the boiler.7. Apparatus in accordance claim 1, wherein the reactor is fitted with acomponent which functions as a catalyst to the cracking in the reactor.8. Apparatus in accordance with claim 1, wherein the reactor is formedof a high-temperature steel alloy containing nickel, which nickelfunctions as a catalyst to the cracking in the reactor.
 9. Apparatus inaccordance with claim 1, wherein the reactor is lined withhigh-temperature refractory.
 10. Apparatus in accordance with claim 5,wherein said fan is positioned at the outlet from the boiler orevaporator.